1. Field
This disclosure is generally related to percutaneous cardiac surgery, and more particularly to percutaneously deployed medical devices suitable for determining locations of cardiac features and/or ablating regions of cardiac tissue.
2. Description of the Related Art
Cardiac surgery was initially undertaken only by performing a sternotomy, a type of incision in the center of the chest, that separates the sternum (chestbone) to allow access to the heart. In the previous several decades, more and more cardiac operations are performed using percutaneous techniques, that is medical procedures where access to inner organs or other tissue is gained via a catheter.
Percutaneous surgeries benefit patients by reducing surgery risk, complications and recovery time. However, the use of percutaneous technologies also raises some particular challenges. Medical devices used in percutaneous surgery need to be deployed via narrow tubes called catheter sheaths, which significantly increase the complexity of the device structure. As well, doctors do not have direct visual contact with the medical tools used once they are placed within the body, and positioning the tools correctly and operating the tools successfully can often be very challenging.
One example of where percutaneous medical techniques are starting to be used is in the treatment of a heart disorder called atrial fibrillation. Atrial fibrillation is a disorder in which spurious electrical signals cause an irregular heart beat. Atrial fibrillation has been treated successfully in open heart methods using a technique know as the “Maze procedure”. During this procedure, doctors create lesions in a specific pattern in the left and right atriums that eliminate the spurious electrical signals. Such lesions were originally created using incisions, but are now typically created by ablating the tissue with RF energy. The procedure is performed with a high success rate under direct vision, but is relatively complex to perform percutaneously because of the difficulty in creating the lesions in the correct spots. Substantial problems, potentially leading to severe adverse results, may occur if the lesions are placed incorrectly.
Key factors which are needed to dramatically improve the percutaneous treatment of atrial fibrillation are enhanced methods for deployment, positioning, and operation of the treatment device. It is particularly important to know the position of the elements which will be creating the lesions relative to cardiac features such as the pulmonary veins and mitral valve.
Several methods have been previously developed for positioning percutaneously deployed medical devices with the heart. However, there are significant challenges associated with each of these methods. One method is to map the inside of the atrium by sensing electrical activity on the atrium wall. Devices that use such a method require intimate electrical contact with the atrium wall which is not always possible because of scar tissue and deposits. Also, such devices fail to accurately map the edges of the openings where the veins enter the atrium, which is important for correct placement of the ablation pattern. Other methods, such as using an array of ultrasonic transducers, are not practical as devices that make use of such methods will not fit through a catheter of a reasonable size (6-8 mm diameter). Yet another method for positioning the treatment device is to make use of an external system for providing navigation, such as a magnetic positioning system. These systems are very expensive and have difficulty delivering the resolution and accuracy needed for correct placement of ablation.
Atrial fibrillation is but one example of a cardiac surgery that requires improved navigation and deployment for percutaneous treatment. There are many others that require similar improvement, such as mitral valve repair.
Thus, there is a need for methods and apparatus that improve navigation and percutaneous deployment of medical devices, as well as determination of the relative position of cardiac features such as pulmonary veins and the mitral valve with respect to a medical device. There is a further need for methods and apparatus that allow the formation of lesions in a specified position relative to cardiac features such as pulmonary veins and the mitral valve.